Method for production of high purity silicon

ABSTRACT

The invention relates to a method for the production of high purity silicon, characterized by the following steps: a) reaction of metallic silicon with silicon tetrachloride (SiCl 4 ), hydrogen (H 2 ) and hydrochloric acid (HCl) at a temperature of 500 to 800° C. and a pressure of 25 to 40 bar to give a trichlorosilane-containing (SiHCl 3 ) feed gas stream, b) removal of impurities from the resultant trichlorosilane-containing feed gas stream by scrubbing with condensed chlorosilanes at a pressure of 25 to 40 bar and a temperature of 160 to 200° C. in a multi-stage distillation column, to give a purified trichlorosilane-containing feed gas stream and a solid-containing chlorosilane suspension and a distillative separation of the purified feed gas stream into a partial stream essentially comprising SiCl 4  and a partial stream, essentially comprising SiHCl 3 , c) disproportionation of the SiHCl 3 -containing partial stream to give SiCl 4  and SiH 4 , whereby the disproportionation is carried out in several reactive/distillative reaction zones, with a counter-current of vapour and liquid, on catalytic solids at a pressure of 500 mbar to 50 bar and SiHCl 3  is introduced into a first reaction zone, the lower boiling SiH 4 -containing disproportionation product produced there undergoes an intermediate condensation in a temperature range of −25° C. to 50° C., the non-condensing SiH 4 -containing product mixture is fed to one or more further reactive/distillative reaction zones and the lower boiling product thus generated, containing a high proportion of SiH 4  is completely or partially condensed in the head condenser and d) thermal decomposition of the SiH 4  to give high purity silicon.

[0001] The present invention relates to a method for producinghigh-purity silicon by reaction of metallic silicon with silicontetrachloride (SiCl₄), hydrogen (H₂), and hydrochloric acid (HCl),removal of impurities from the resultant trichlorosilane-containing(SiHCl₃) feed gas stream, disproportionation of SiHCl₃ to give SiCl₄ andsilane (SiH₄), and thermal decomposition of the SiH₄ to give high-puritysilicon.

[0002] High-purity silicon is needed in the production of semiconductorsand solar cells. In this context, the required purity level of thesilicon is very high. The production of high-purity silicon meetingthese requirements is performed in accordance with “Silicon for theChemical Industry IV, Geiranger, Norway, Jun. 3-5, 1998, Ed.: H. A. Øye,H. M. Rong, L. Nygaard, G. Schüssler, J. Kr. Tuset, pp. 93-112”following three different methods:

[0003] reaction of metallic silicon with hydrochloric acid (HCl) in afluid bed to give SiHCl₃, purification of SiHCl₃, thermal decompositionof the purified SiHCl₃ in the presence of H₂ on silicon bars to givehigh-purity silicon. The SiCl₄-containing reaction gas generated duringthe thermal decomposition of SiHCl₃ is condensed and purified. It is adrawback of this method that large quantities of SiCl₄ are produced as aby-product which either reacts in a separate, technically andenergetically very expensive process step to give SiHCl₃ or has to beutilised, for example in the production of pyrogenic silicic acid.

[0004] Reaction of silicon tetrafluoride (SiF₄) with sodium-aluminiumhydride (NaAlH₄) to give SiH₄ and sodium-aluminium fluoride (NaAlF₄),purification of the resultant SiH₄, separation of high-purity silicon onsilicon seed particles in a fluid bed, and removal of H₂ from thegenerated granulated high-purity silicon. Large quantities of NaAlF₄ areproduced which have to be utilised or sold. Another drawback of thismethod is that during the separation of high-purity silicon in the fluidbed a considerable amount of high-purity silicon powder is producedwhich has to be disposed of as there is no way of economic utilisation.

[0005] Reaction of metallic silicon with SiCl₄ and H₂ in a fluid bed togive SiHCl₃, catalysed 2-stage disproportionation of SiHCl₃ to giveSiCl₄ and SiH₄, feedback of the generated SiCl₄ into the reaction ofmetallic silicon with SiCl₄ and H₂, thermal decomposition of thegenerated SiH₄ on silicon bars to give high-purity silicon and feedbackof the H₂ generated during this process into the reaction of metallicsilicon with SiCl₄ and H₂.

[0006] The last of the methods described above is characterised by thefact that a forced generation of large quantities of by-products isavoided due to the utilisation of SiCl₄ generated during the process inthe production of SiHCl₃, i.e. by reacting it with metallic silicon andhydrogen.

[0007] Embodiments of this method are described in “Studies in OrganicChemistry 49, Catalyzed Direct Reactions of Silicon, Elsevier, 1993, pp.450 through 457” and DE 3 311 650 C2 and CA-A-1 162 028. According tothese documents, the production of high-purity silicon following thismethod comprises the following steps:

[0008] 1. Reaction of metallic silicon with SiCl₄ and H₂ at 400 to 600°C. and a pressure of 20.7 to 41.4 bar in a fluid bed reactor.

[0009] 2. Removal of impurities such as non-reacted fine-grain silicon,metal chlorides and catalyst, if applicable, from the generatedchlorosilane-containing and hydrogen-containing reaction mixture byscrubbing the hot gas stream with condensed chlorosilanes.

[0010] 3. Disposal of the solid-containing chlorosilane suspension thusgenerated.

[0011] 4. Condensation of the purified reaction mixture.

[0012] 5. Feedback of the hydrogen generated in step 4 to step 1.

[0013] 6. Distillative separation of the purified reaction mixture togive SiCl₄ and SiHCl₃.

[0014] 7. Feedback of SiCl₄ to step 1.

[0015] 8. 2-stage catalysed disproportionation of SiHCl₃ generated instep 6 to give SiH₄ and SiCl₄.

[0016] 9. Feedback of SiCl₄ to step 1.

[0017] 10. Distillative purification of SiH₄ generated in step 8.

[0018] 11. Thermal decomposition of the purified SiH₄ while generatinghigh-purity silicon and hydrogen.

[0019] 12. Utilisation of the hydrogen thus generated as thinning gas instep 11 or feedback to step 1.

[0020] During the execution of step 1, the yields obtained fallsignificantly short of the yields to be expected in the light of thethermodynamic equilibrium due to the slow reaction speed. In order toaccelerate the reaction, DE 3 311 650 C2 and DE 4 104 422 A1 suggest theuse of copper catalysts. The copper catalysts are usually fed into thefluid bed together with the ground silicon or separately. The drawbackof this method is that in the fluid bed a portion of the catalyst isdirectly discharged from the fluid bed together with the gaseousreactants and/or the reaction products due to insufficient adhesion tothe silicon particles and thus is no longer available for the reaction.This causes a considerable need of copper catalyst adversely affectingthe economic efficiency of this method with a view to the normally highprice of copper catalysts.

[0021] During the reaction of metallic silicon with SiCl₄ and hydrogen,chlorides such as aluminium chloride (AlCl₃), iron chloride (FeCl₂) andcalcium chloride (CaCl₂) are generated in addition to the desiredchlorosilanes. Most of these metal chlorides are removed as crystallisedsolid particles in step 2 by scrubbing the hot gas stream with condensedchlorosilanes. Due to the high sublimation pressure, crystallised AlCl₃may, however, be distributed via the gas chamber in the entire scrubbingfacility. This causes deposition of AlCl₃ on the scrubbing facility andits internal components so that periodic shutdowns and cleaning measuresof the scrubbing facility are required.

[0022] The removal of AlCl₃ by distilling the chlorosilanes can beperformed at lowered temperatures in a vacuum. Under these conditions,the vapour pressure of the solid aluminium chloride is so low that itsshare in the gaseous phase may fall below the solubility limit so thatthe problem of deposition of solid aluminium chloride in the facility isavoided. However, it is not possible to lower the AlCl₃ portion belowthe share of its vapour pressure in the entire vapour pressure so thaton the one hand the separation of AlCl₃ is possible only to a limiteddegree while on the other hand due to this fact the problem of aluminiumchloride deposition during another distillation process occurs anew,although at lesser quantities. The method described in DE 2 161 641 A1offers a partial solution of this problem wherein the gas stream leavingthe synthesis reactor is cooled in two stages by firstly feeding SiCl₄to reach 250 to 300° C. and secondly by using a Venturi scrubber withmore SiCl₄ to reach approx. 53° C., wherein iron chloride and aluminiumchloride carried by the chlorosilanes are deposited and remain in thecondensation product while the gas stream containing the major portionof the chlorosilanes is again scrubbed with condensed chlorosilanes sothat it can be condensed thereafter. The remaining content of AlCl₃ isthen determined by the vapour pressure of AlCl₃ at approx. 53° C. Duringthe further purification of the chlorosilanes, the aforementionedproblem of AlCl₃ concentration in the sump occurs again, causing theformation of a solid phase of AlCl₃ and its undesired sublimation. Quitesimilarly, the method described in DE 2 623 290 A1 offers only a partialsolution wherein the gas stream containing the chlorosilanes and AlCl₃is cooled down to 60 to 80° C. with the purpose of letting the solidAlCl₃ deposit and separating it.

[0023] The removal of the metallic silicon and metallicchloride-containing chlorosilane suspension generated in step 2 isperformed in accordance with DE 3 709 577 A1 by a specific distillativeseparation of the chlorosilanes from the solid particles wherein a largeportion of the chlorosilanes can be recovered. The remaining solid andchlorosilane-containing distillation sump cannot be utilised and thushas to be disposed of as described for example in U.S. Pat. No.4,690,810. This process has an adverse effect on the economy of thismethod. Another drawback is the fact that disturbing impurities are,together with the recovered chlorosilanes, fed back into the process ofproducing high-purity silicon which may cause an undesired concentrationof these impurities and hence adversely affect the process.

[0024] During the steps 4, 6, 8 and 10, silane andchlorosilane-containing waste streams are generated which, as a rule,are disposed of by scrubbing with solutions of alkali compounds or, forexample, by combustion. Several variants of the method for disposing ofthe waste streams are for example described in U.S. Pat. No. 4,519,999.In these methods, the reactive silicon compounds are made harmless byconverting them into silicates or silicic acids. However, the drawbackof this method is that the actually valuable waste stream componentssuch as SiH₄, SiH₂Cl₂, SiHCl₃ and SiCl₄ are converted into less valuableproducts.

[0025] The 2-stage catalysed disproportionation of SiHCl₃ to give SiH₄and SiCl₄ performed in step 8 requires very expensive equipment andpower. According to DE 2 507 864 A1, an improved variant of the methodis to have the disproportionation of SiHCl₃ take place in a single stepfollowing the principle of reactive distillation. There is, however, agrave disadvantage in this method described in DE 2 507 864 A1, namely,that the amount of energy used for the separation of the silanes orchlorosilanes has to be completely discharged on a very low temperaturelevel of less than −50° C. to −120° C. corresponding to the condensationtemperatures. It is known that heat discharge on a low temperature levelis expensive and implies high power consumption.

[0026] The thermal decomposition of purified SiH₄ of step 11 may beperformed following three different methods according to DE 3 311 650C2:

[0027] A. Semi-continuous deposition of high-purity silicon on siliconbars, known as Siemens method.

[0028] B. Continuous decomposition of SiH₄ in a gaseous phase reactorincluding the generation of powder-like high-purity silicon.

[0029] C. Continuous decomposition of SiH₄ on silicon seed particles ina fluid bed including the generation of granulated high-purity silicon.

[0030] The method variant A requires very expensive equipment and, dueto the inevitable semi-continuous operation, regular plant shutdownswith expensive cleaning work. In the method variant B, high-puritysilicon is generated as a fine powder which cannot be directly utilised,so that this powder has to be compressed and molten in expensivesubsequent steps. Further, this high-purity silicon powder is easilycontaminated due to its very large specific surface and therefore cannormally not be utilised in the fields of photovoltaics or semiconductortechnology.

[0031] In the method variant C, however, granulated high purity-siliconis continuously produced which can easily be processed further. Variantsof this method are described, for example, in U.S. Pat. No. 3,012,861and U.S. Pat. No. 5,798,137. According to these documents, the drawbacksof the method variant C are that a considerable portion of the SiH₄ usedis decomposed to powder-like high-purity silicon. In order to lessen thepowder production, SiH₄ decomposition may be performed by addinghydrogen at nearly ambient pressure.

[0032] The high-purity silicon powder is, as described above, easilycontaminated due to its very large specific surface when it is furtherprocessed in subsequent steps and therefore cannot be utilised in thefields of photovoltaics or semiconductor technology, either.

[0033] The drawbacks mentioned above are the reason why the productionof high-purity silicon is still very expensive so that in particular theeconomic profitability and with it the growth of photovoltaicapplications using high-purity silicon are considerably affected.

[0034] It is therefore the object of the present invention to provide amethod for producing high-purity silicon which does not imply thedrawbacks mentioned above and allows a cost-effective production.

[0035] The present invention relates to the production of high-puritysilicon, characterised by the following steps:

[0036] a) reaction of metallic silicon with silicon tetrachloride(SiCl₄), hydrogen (H₂) and hydrochloric acid (HCl) at a temperature of500 to 800° C. and a pressure of 25 to 40 bar to give atrichlorosilane-containing (SiHCl₃) feed gas stream,

[0037] b) removal of impurities from the resultanttrichlorosilane-containing feed gas stream by scrubbing with condensedchlorosilanes at a pressure of 25 to 40 bar and a temperature from 160to 220° C. in a multi-stage distillation column, to give a purifiedtrichlorosilane-containing feed gas stream and a solid-containingchlorosilane suspension, and distillative separation of the purifiedfeed gas stream into a partial stream essentially consisting of SiCl₄and a partial stream essentially consisting of SiHCl₃,

[0038] c) disproportionation of the SiHCl₃-containing partial stream togive SiCl₄ and SiH₄, whereby disproportionation is carried out inseveral reactive/distillative reaction zones, with a counter-current ofvapour and liquid, on catalytically active solids at a pressure of 500mbar to 50 bar and SiHCl₃ is introduced into a first reaction zone, thelower-boiling SiH₄-containing disproportionation product produced thereundergoes an intermediate condensation in a temperature range of −25 to50° C., the non-condensed SiH₄-containing product mixture is fed to oneor more further reactive/distillative reaction zones and the lowerboiling point product thus generated, containing a high proportion ofSiH₄ is completely or partially condensed in the top condenser, and

[0039] d) thermal decomposition of SiH₄ to give high-purity silicon.

[0040] Here, high-purity silicon means silicon having a purity highenough to be suitable in the field of photovoltaics. This requires allmetal concentrations to remain below 0.1 ppm, carbon content below 1ppm, oxygen below 5 ppm, phosphorus below 0.1 ppm, and boron below 0.05ppm.

[0041] Metallic silicon means silicon which may contain up toapproximately 3 wt. % iron, 0.75 wt. % aluminium, 0.5 wt. % calcium, andother impurities as being usually found in silicon and which waspreferably obtained by carbo-thermic reduction of silicon dioxide.

[0042] It was found that the execution of the method according to thepresent invention brings about a number of advantages allowing asignificantly more cost-effective production of high-purity silicon.

[0043] In the reaction of metallic silicon with SiCl₄, H₂ and HCl at atemperature of 500 to 800° C., preferably 550 to 650° C., and a pressureof 25 to 40 bar, preferably 30 to 38 bar, high space/time yields ofSiHCl₃ are obtained.

[0044] The addition of hydrochloric acid, preferably at an amount of0.05 to 10 wt. %, most preferably 1 to 3 wt. %, each being relative tothe amount of SiCl₄ added as additional reactant, causes an unexpectedacceleration of the reaction which finally leads to the fact that verylarge SiHCl₃ yields, i.e. high reaction levels of the SiCl₄ used nearthe thermodynamic equilibrium, and at the same time high total yields,i.e. large-scale utilisation of the metallic silicon used, are achieved.

[0045] Hydrochloric acid is preferably used in its water-free form ashydrochloric gas.

[0046] Hydrochloric acid can, for example, be separately fed into thereactor in which the reaction to produce chlorosilane is to beperformed. However, it is also possible to feed hydrochloric acidtogether with the gaseous and/or evaporable initial substances to be fedinto the reactor, namely hydrogen and/or silicon tetrachloride.

[0047] The selection of the reactor in which the reaction according tothe present invention is to be performed is not critical as long as thereactor ensures enough stability with respect to the reaction conditionsand allows the contact of the initial substances. Applicable reactorsare, for example, a solid bed reactor, a rotating tube furnace or afluid bed reactor. Preferably the reaction is performed in a fluid bedreactor.

[0048] The material of the reactor has to withstand the reactionconditions mentioned with respect to SiHCl₃ synthesis. The requirementsregarding the durability of the structural reactor materials areapplicable as well to possible up- or downstream facility componentssuch as cyclones or heat exchangers. These requirements are met, forexample, by nickel-based alloys.

[0049] The use of catalysts allows an additional acceleration of thereaction of metallic silicon with SiCl₄, H₂ and HCl. Especially suitablecatalysts are copper, iron, copper or iron compounds or mixturesthereof.

[0050] Surprisingly, it became evident that the catalysts wereparticularly effective when the metallic silicon was grounded andthoroughly mixed with the catalysts prior to the reaction.

[0051] In the method according to the present invention, the reaction togive trichlorosilane (step a)) is preferably performed in the presenceof a catalyst, wherein the metallic silicon is thoroughly mixed with thecatalyst prior to the reaction.

[0052] Preferably, silicon having mean grain diameters between 10 and1000 μm, preferably 100 to 600 μm, is used. The mean grain diameter isdetermined as the numerical average of the values obtained in a sievinganalysis of the silicon.

[0053] For mixing the catalyst and the silicon, devices are preferablyused which ensure very thorough mixing. Mixers having rotating mix toolsare most suitable for this purpose. Such mixers are described, forexample, in “Ullmann's Encyclopedia of Industrial Chemistry, Volume B2,Unit Operations 1, pp. 27-1 to 27-16, VCH Verlagsgesellschaft,Weinheim”. Most preferably, ploughshare mixers are used.

[0054] During the thorough mixing, the catalyst may be crushed furtherso as to ensure during the mixing process a very good distribution and avery good adhesion of the catalyst to the silicon surface. Thus, evencatalysts which are not available as fine particles and/or cannot becrushed to the fineness required may be used.

[0055] In case of insufficient mixing, a major portion of the catalystis, due to its low adhesion to the silicon particles, directlydischarged from the fluid bed together with the gaseous reactants and/orproducts and thus is no longer available for the reaction. This raisesthe need for catalyst and adversely affects the profitability of themethod. This is prevented by thoroughly mixing the silicon and thecatalyst.

[0056] The time period for mixing the silicon and the catalyst ispreferably 1 to 60 minutes. Longer mixing times are normallyunnecessary. Mixing times of 5 to 20 minutes are particularly preferred.

[0057] The thorough mixing of the catalyst and the silicon may beperformed, for example, in an inert atmosphere or in the presence ofhydrogen or other gases having a reducing effect such as carbonmonoxide. Among other effects, this prevents the formation of an oxidiclayer on the individual silicon particles. Such a layer prevents thedirect contact between the catalyst and the silicon and thus impairs thecatalysed reaction of the latter with silicon tetrachloride, hydrogenand, if applicable, hydrochloric acid to give trichlorosilane.

[0058] An inert atmosphere may for example be generated by adding aninert gas during the mixing process. Suitable inert gases include forexample nitrogen and/or argon.

[0059] The mixing of silicon and catalyst is preferably performed in thepresence of hydrogen.

[0060] In principle, any catalyst known for the reaction of silicon withsilicon tetrachloride, hydrogen and, if applicable, hydrochloric acidmay be used.

[0061] Particularly suitable catalysts are copper catalysts and ironcatalysts. Examples include copper oxide catalysts (e.g. Cuprokat® ofMessrs. Norddeutsche Affinerie), copper chloride (CuCl, CuCl₂), coppermetal, iron oxides (e.g. Fe₂O₃, Fe₃O₄), iron chlorides (FeCl₂, FeCl₃)and mixtures thereof.

[0062] Preferred catalysts are copper oxide catalysts and iron oxidecatalysts.

[0063] Especially during the use of copper oxide catalysts and ironoxide catalysts, it has proven to be advantageous to perform the mixingwith silicon at a temperature of 100 to 400° C., preferably 130 to 350°C. When doing so, remaining moisture adhering to the catalysts areremoved which adversely affects the reaction of silicon with SiCl₄, H₂and, if applicable, HCl. Moreover, this approach ensures a betteradhesion of the catalyst to the silicon surface thus largely avoidingloss of catalyst in the fluid bed.

[0064] It is also possible to use mixtures of copper and/or ironcatalysts with other catalytically active components. Such catalyticallyactive components include, for example, metal halogenides such aschlorides, bromides or iodides of aluminium, vanadium or antimony.

[0065] The quantity of catalyst used, calculated as metal, is preferably0.5 to 10 wt. %, most preferably 1 to 5 wt. % relative to the quantityof silicon used.

[0066] As an alternative, the method according to the present inventionallows in the reaction to give trichlorosilane (step a)) also metallicsilicon having an iron content of 1 to 10 wt. %, preferably 1 to 5 wt. %to be used, wherein the iron is mostly homogeneously distributed in themetallic silicon, preferably in the form of a silicide.

[0067] Silicon containing homogeneously distributed iron may for examplebe produced by melting a mixture of silicon and the desired quantity ofiron or by adding a desired quantity of iron to a silicon melt, followedby rapid cooling of the melt. Preferably, the addition of the desiredquantity of iron is performed as early as during the production of thesilicon.

[0068] The rapid cooling of the melt may be performed, for example, byjetting the melt in air or by water granulation.

[0069] The preferred method for rapid cooling of a silicon melt and thusfor producing usable silicon is water granulation. During the watergranulation, liquid silicon is fed into water. This causes the siliconto cool down at extreme speed. Depending upon the process parameters, itis possible for example to obtain silicon pellets. Water granulation ofsilicon is known for example from EP 522 844 A2.

[0070] Then the iron is present in the silicon in fine particles beinghomogeneously distributed.

[0071] The mol ratio of hydrogen and silicon tetrachloride may be, forexample, 0.25:1 to 4:1 in step a) of the method according to the presentinvention. The preferred mol ratio is 0.6:1 to 2:1.

[0072] According to the present invention, thetrichlorosilane-containing feed gas stream generated in the reaction ofmetallic silicon with SiCl₄, H₂ and HCl (step a)) is purified byscrubbing with condensed chlorosilanes at a pressure of 25 to 40 bar,preferably 35 to 40 bar, and a temperature of 160 to 220° C., preferably190 to 200° C., in a multi-stage distillation column (step b)).

[0073] Suitable condensed chlorosilanes include, for example, acondensed gas stream comprising trichlorosilane and silicontetrachloride at a mol ratio of approximately 1:3 to 1:20.

[0074] Surprisingly, it became evident that, when the aforementionedtemperature and pressure ranges are observed, the silicon powderremnants and metal chlorides, especially AlCl₃, which may be containedin the trichlorosilane-containing feed gas stream are completelyseparated from the feed gas stream and can be removed with the condensedchlorosilanes from the scrubbing column as solids or as dissolved metalchloride (e.g. AlCl₃).

[0075] The problems described above which are caused by sublimation ofAlCl₃ in the scrubbing column are surely avoided in the approachaccording to the present invention. This ensures a faultless long-termoperation of the scrubbing column and thus the entire process. Anysubstances still present having a higher boiling point such asdisilanes, polysilanes, siloxanes and hydrocarbons are removed from thefeed gas stream together with the condensed chlorosilanes.

[0076] A chlorosilane suspension is produced which can then be relaxedand cooled down wherein dissolved metal chlorides, especially AlCl₃,fall out almost completely except a few ppm.

[0077] Following relaxation and cooling, solids are preferably removedfrom the chlorosilane suspension by filtration. The solid-freechlorosilanes may be transferred to utilisation while treating theseparated solids with alkali compounds.

[0078] The filtration of the chlorosilane suspension is preferablyperformed using sinter metal filter substances. Such filters are knownand described, for example, in “Ullmanns Encyklopädie der technischenChemie, 4^(th) edition, Vol. 19, p. 573, Verlag Chemie, 1980”.

[0079] The solid-free filtrate is an extremely suitable raw material forthe production of pyrogenic silicic acid. Further processing, forexample, by distillation, is not required. The solids produced duringfiltration may be made inert in a known manner using alkali compoundssuch as soda lye, Na₂CO₃, NaHCO₃ and CaO and used after inertisation asraw material for cement production.

[0080] In an advantageous variant of the method according to the presentinvention, the trichlorosilane-containing feed gas is made free from anyexisting powder-like solids by gas filtration prior to scrubbing withcondensed chlorosilanes. This can for example be performed in cyclones,wherein several cyclones connected in series and/or one or moremulti-cyclones can be used to achieve high separation levels. As analternative, hot-gas filters with sinter metal or ceramic candles orcombinations of cyclones and hot-gas filters may be used. This approachhas the advantage that the subsequent feed gas scrubbing issignificantly facilitated while a silicon metal-containing solid isobtained which, due to its high content of silicon, can be transferredto utilisation in metallurgical processes such as the production of ironalloys. For this purpose, the silicon metal and metalchloride-containing solid may for example react with alkali compoundssuch as soda lye, Na₂CO₃, NaHCO₃ and CaO and water, be filtrated andwashed with water to remove chloride and then be dried, if required.

[0081] The now purified feed gas stream is condensed in a known mannerand separated by preferably multi-stage distillation into a partialstream mainly consisting of SiCl₄ and a partial stream mainly consistingof SiHCl₃.

[0082] Preferably, the partial stream mainly containing SiCl₄ is fedback into the reaction of metallic silicon with silicon tetrachloride,H₂ and HCl (step a)).

[0083] Distillation may be performed at a pressure of 1 to 40 bar.Preferably distillation is performed at a pressure of less than 10 barin order to achieve a good separation of SiCl₄ and SiHCl₃ with a minimumof distillation stages.

[0084] According to the present invention, the SiHCl₃-containing partialstream is fed to a subsequent disproportionation. It has been proven asadvantageous to remove most of the components which have a lower boilingpoint than SiHCl₃ from this partial stream in a multi-stage distillationprocess. This distillation may also be performed at a pressure of 1 to40 bar. Preferably, the distillation is performed at a pressure of lessthan 10 bar in order to achieve a good separation of compounds having alower boiling point from SiHCl₃ with a minimum of distillation stages.

[0085] In another advantageous variant, the purified SiHCl₃ issubsequently made free from anhydric acids such as halogenides andhydrides using caustic solids. Examples of anhydric acids include BCl₃,BH₃, PCl₃, HCl. The advantage is that the efficiency of the subsequentcatalysed disproportionation is not adversely affected so that along-term faultless operation of the disproportionation process isensured. The caustic solids used may be identical with thedisproportionation catalysts used in the following step.

[0086] The contact with caustic solids may be performed, for example, ina solid bed reactor. The process is preferably performed at a pressureof 1 to 50 bar, most preferably 1 to 10 bar. The temperatures may forexample be in the range from 30 to 180° C., preferably 50 to 110° C. Thetemperature to be set depends upon the stability range of the causticsolids used. In order to ensure continuous operation, two or morereactors provided with caustic solids can be connected in parallel. Itis possible to regularly switch over to a reactor filled with freshsolids to ensure a complete removal of the aforementioned impuritieswhile the consumed solids are exchanged and regenerated, if required.Similarly, a reactor can be operated as several reactors connected inseries.

[0087] The disproportionation of the purified, if required,trichlorosilane-containing partial stream (step c)) is most preferablyperformed in a column at a pressure of 1 to 10 bar, wherein said columncomprises at least two reactive/distillative reaction zones.

[0088] Disproportionation takes place on catalytically active solids,preferably in catalyst beds each comprising a bulk solid layerconsisting of solid pieces working as catalyst solids, wherein thedisproportionation products are able to flow through this layer. In thereaction zone, the bulk solid layer may be replaced by packed catalystbodies.

[0089] Suitable catalytically active solids are known and described, forexample, in DE 2 507 864 A1. Such suitable solids are, for example,solids which carry amino or alkyleneamino groups on a polystyrenestructure meshed with divinyl benzene. Amino or alkyleneamino groupsinclude for example: dimethylamino, diethylamino, ethylmethylamino,di-n-propylamino, di-iso-propylamino, di-2-chloroethylamino,di-2-chloropropylamino groups as well as the similarly substituteddialkylaminomethylene groups and the corresponding hydrochlorides or thetrialkylammonia groups derived therefrom by methylisation, ethylisation,propylisation, butylisation, hydroxyethylisation or benzylisation withchloride as counter-ion. Of course, in the case of quaternary ammoniasalts or protonised ammonia salts, catalytically active solids havingother anions, e.g. hydroxyde, sulphate, hydrogen sulphate, bicarbonateand the like, may be fed into the method according to the presentinvention, although a gradual conversion into the chloride form isinevitable under the reaction conditions, which applies to organichydroxy groups as well. Therefore, ammonia salts containing chloride ascounter-ion are preferred.

[0090] Other suitable catalytically active solids include, for example,solids consisting of a polyacrylic acid structure, especially apolyacrylamide structure, which has bonded trialkylbenzyl ammonia, forexample, via an alkyl group.

[0091] Another suitable group of catalytically active solids includes,for example, solids having bonded sulphonate groups to a polystyrenestructure, meshed with divinyl benzene, being confronted by tertiary orquaternary ammonia groups as kations.

[0092] As a rule, macroporous or mesoporous exchanger resins are moresuitable than gel resins.

[0093] The trichlorosilane-containing partial stream of step b) is fedto the reaction column through an inlet opening into the column at anappropriate point. Such an appropriate point is, for example, a point atwhich the column has an inner temperature corresponding to the boilingpoint of trichlorosilane at the existing pressure. In the reactionzones, a SiH₄-containing, vapour-like product mixture ascending in thereaction zone and a SiCl₄-containing liquid mixture descending in thereaction zone are formed by disproportionation of SiHCl₃.

[0094] The SiCl₄-containing liquid flowing out of the reaction zone isfed inside the reaction column into a distillative depression unitbeneath the reactive/distillative zone from which unit silicontetrachloride SiCl₄ may flow off as sump product.

[0095] For the SiH₄-containing product mixture ascending in the reactionzone, an intermediate condenser is provided above the reaction zone inwhich condenser the concentration of SiH₄ in the product mixture isincreased by partial condensation of components having a higher boilingpoint at a temperature between −25° C. and 50° C., preferably between−5° C. and 40° C. The product components having a lower boiling pointwhich were not condensed in the intermediate condenser are supplied fora further concentration increase to a second reactive/distillativereaction zone downstream the intermediate condenser in the flowdirection of the ascending product components and then to a boosterunit.

[0096] Preferably, disproportionation is performed so that severalintermediate condensation processes take place in the reaction zones asa whole on different temperature levels ranging from −25° C. to 50° C.

[0097] The use of three or more reactive/distillative reaction zones andtwo or more intermediate condensers allows the discharge of theintermediate condensation heat at different temperature levels with lowdriving temperature differences with advantageously low powerconsumption.

[0098] The booster unit may be arranged inside or outside the reactioncolumn. The product mixture leaving the booster unit is finally suppliedto a top condenser where it is deposited and discharged. A portion ofthe product mixture may be fed back to the top of the reaction column.

[0099] Impurities depositing at different temperature levels in thereaction column may be taken out of the column via lateral removalpoints.

[0100] In order to further lower the condensation energy to bedischarged at a very low temperature, the feedback quantity can bedecreased and a top product be generated having a lesser silane purityof between 25% and 90%. This top product may then be separated to befurther purified in a downstream separation column wherein an equal orpreferably higher pressure than in the reaction column, preferably 15bar to 100 bar, is set so that the separation column operates at highertemperatures than the reaction column with respect to an equalcomposition. The sump product of the separation column may, dependingupon the selected operating conditions, contain large quantities oftrichlorosilane, dichlorosilane and monochlorosilane. The sump productmay entirely or partly be fed back into the reaction column. Impuritiesmay, if required, be removed from the system by sluicing out a partialstream.

[0101] Preferably, SiCl₄ obtained during disproportionation is fed backinto the reaction of silicon with SiCl₄, H₂ and HCl (step a)).

[0102] According to the present invention, SiH₄ obtained duringdisproportionation is thermally decomposed (step d)).

[0103] It is possible to subject SiH₄ obtained during disproportionationto a distillative purification prior to its thermal decomposition.

[0104] In a particularly preferred embodiment of the method according tothe present invention, the thermal decomposition of SiH₄ which has beenpurified by distillation, if required, is performed on high-puritysilicon seed particles in a fluid bed at a pressure of 100 to 900 mbar.

[0105] Thermal decomposition is preferably performed at pressures from200 to 800 mbar. The pressure range between 300 and 700 mbar, mostpreferably between 400 and 600 mbar, is particularly preferred. Allspecified pressure values are absolute pressure values. Theaforementioned pressure means the pressure existing behind the fluid bedas viewed in the flow direction of the gas stream supplied.

[0106] In the thermal decomposition of silane, it is possible to add upto 50 vol. % of a silicon-free gas with relation to the entire gassupplied. Preferably, the added quantity of silicon-free gas is 0 to 40vol. %, most preferably 0 to 20 vol. %. The addition of silicon-free gasreduces the formation of silicon powder.

[0107] Suitable silicon-free gases include, for example, the rare gases,nitrogen and hydrogen, wherein the silicon-free gases may be usedindividually or in any combination thereof. Nitrogen and hydrogen arepreferred, with hydrogen being preferred most.

[0108] The advantageous temperature range for the thermal decompositionof silane is between 500° C. and 1400° C. A decomposition temperature of600° C. to 1000° C. is preferred, with 620° C. to 700° C. beingpreferred most.

[0109] The high-purity silicon seed particles may be fed into thereaction chamber of a fluid bed reactor. The high-purity silicon seedparticles may be fed from outside intermittently or continuously.However, particles being generated in the reaction chamber may be usedas high-purity silicon seed particles as well. The high-purity seedparticles form a solid bed through which the supplied gas is blown in.The blow-in speed of the supplied gas is set so that the solid bed isfluidised and a fluid bed is formed. The relevant approach as such isknown to a person skilled in the art. The blow-in speed of the suppliedgas has to be at least equal to the loosening speed. Loosening speedmeans the speed at which a gas flows through a particle bed and belowwhich the solid bed is retained, i.e. below which the bed particlesremain mostly immobile. Above this speed, fluidisation of the bedstarts, i.e. the bed particles move, and initial bubbles are formed.

[0110] In this embodiment, the blow-in speed of the supplied gas is oneto ten times, preferably one-and-a-half to seven times, the looseningspeed.

[0111] The high-purity seed particles being advantageously used havediameters between 50 and 5000 μm.

[0112] The high-purity seed particles may, for example, be generated bycrushing the granulated high-purity silicon generated during thermaldecomposition of SiH₄ in the fluid bed. Usual crushing methods such asgrinding imply the risk that the high-purity silicon seed particles arecontaminated during the crushing process.

[0113] Therefore, the production of the high-purity silicon seedparticles is preferably performed by their generation in thedecomposition reactor itself, separation of a fraction of appropriateparticle size by process-internal inspection and their feedback into thereactor.

[0114] The hydrogen generated during thermal decomposition of silane ispreferably fed back into the reaction of silicon with SiCl₄, H₂ and HCl(step a)).

[0115] In another preferred variant, the high-purity silicon powdergenerated as a by-product during thermal decomposition of SiH₄ is,following its separation from the granulated high-purity silicon, in aseparate process step heated up my microwave irradiation at wavelengthsbetween 0.5 kHz and 300 GHz to a temperature of at least 300° C. andagglomerated. By doing so, a product is obtained which, without anyfurther processing steps such as condensation and crushing, may forexample be introduced into the melting process for the production ofsilicon wafers for solar cells.

[0116] Another preferred embodiment of the method according to thepresent invention suggests that SiH₄ and SiH₂Cl₂-containing wastestreams of the various distillation processes are collected to reactwith SiCl₄ to give a SiHCl₃-containing reaction mixture from whichSiHCl₃ is obtained by distillation. It is advantageous to use in thisreaction liquid disproportionation catalysts having a boiling pointabove the boiling point of SiCl₄. Suitable disproportionation catalystsinclude, for example, trialkylamines and aryldialkylamines.

[0117] If desired, SiHCl₃ may be used in the disproportionation process(step c) following further purification. Thus, the yield and theprofitability of the entire process are improved because there is noneed for a disposal of the aforementioned waste streams which causes aloss of silicon compounds.

[0118] The high-purity silicon obtained according to the presentinvention may, due to its high purity level, be easily utilised as rawmaterial for the production of semiconductors and solar cells.

[0119] The method according to the present invention allows a verycost-effective production of high-purity silicon due to the utilisationof the waste streams and by-products becoming possible according to thepresent invention, higher yields from the SiHCl₃ synthesis, and theconsiderable lower overall power need.

1. A method for producing high-purity silicon, characterised by thefollowing steps: a) reaction of metallic silicon with silicontetrachloride (SiCl₄), hydrogen (H₂) and hydrochloric acid (HCl) at atemperature of 500 to 800° C. and a pressure of 25 to 40 bar to give atrichlorosilane-containing (SiHCl₃) feed gas stream, b) removal ofimpurities from the resultant trichlorosilane-containing feed gas streamby scrubbing with condensed chlorosilanes at a pressure of 25 to 40 barand a temperature from 160 to 220° C. in a multi-stage distillationcolumn, to give a purified trichlorosilane-containing feed gas streamand a solid-containing chlorosilane suspension, and distillativeseparation of the purified feed gas stream into a partial streamessentially comprising SiCl₄ and a partial stream essentially comprisingSiHCl₃, c) disproportionation of the SiHCl₃-containing partial stream togive SiCl₄ and SiH₄, whereby the disproportionation is carried out inseveral reactive/distillative reaction zones, with a counter-current ofvapour and liquid, on catalytically active solids at a pressure of 500mbar to 50 bar and SiHCl₃ is introduced into a first reaction zone, thelower-boiling SiH₄-containing disproportionation product produced thereundergoes an intermediate condensation in a temperature range of −25 to50° C., the non-condensed SiH₄-containing product mixture is fed to oneor more further reactive/distillative reaction zones and the lowerboiling point product thus generated, containing a high proportion ofSiH₄ is completely or partially condensed in the top condenser, and d)thermal decomposition of the SiH₄ to give high-purity silicon.
 2. Amethod according to claim 1, characterised in that step a) is performedin a fluid bed reactor.
 3. A method according to any of claims 1 and 2,characterised in that hydrochloric acid is introduced at a quantity of0.05 to 10 wt. % relative to the weight of the supplied SiCl₄.
 4. Amethod according to any of claims 1 through 3, characterised in thatstep a) is performed in the presence of a catalyst wherein said catalystand the metallic silicon are thoroughly mixed prior to being fed into areactor.
 5. A method according to claim 4, characterised in that copper,iron, copper compounds, iron compounds, or mixtures thereof are used ascatalyst.
 6. A method according to any of claims 1 through 3,characterised in that metallic silicon having an iron content of 0.5 to10 wt. % is used, wherein iron is mostly homogeneously distributed amongthe metallic silicon.
 7. A method according to any of claims 1 through6, characterised in that solid components, if any, of thetrichlorosilane-containing feed gas stream are separated prior toscrubbing with condensed chlorosilanes.
 8. A method according to any ofclaims 1 through 7, characterised in that the solid-containingchlorosilane suspension generated in step b) is made free of solids byfiltration, the solid-free chlorosilanes are transferred to furtherutilisation, and the solids are treated with alkali compounds.
 9. Amethod according to claim 8, characterised in that filtration isperformed using sinter metal filter materials.
 10. A method according toany of claims 1 through 9, characterised in that the SiCl₄-containingpartial stream obtained in step b) is fed back into the reaction ofmetallic silicon with SiCl₄, H₂ and HCl (step a)).
 11. A methodaccording to any of claims 1 through 10, characterised in that thepartial stream consisting mostly of trichlorosilane obtained in step b)is made mostly free from components having a lower boiling point thanSiHCl₃ prior to disproportionation.
 12. A method according to any ofclaims 1 through 11, characterised in that the partial stream consistingmostly of trichlorosilane obtained in step b) is brought into contactwith caustic solids prior to disproportionation for the purpose ofremoving anhydric acids such as halogenides and hydrides.
 13. A methodaccording to any of claims 1 through 12, characterised in thatdisproportionation of trichlorosilane (step c)) is performed at apressure between 1 and 10 bar.
 14. A method according to any of claims 1through 13, characterised in that in step c) several intermediatecondensation processes at different temperature levels in the range from−25° C. to 50° C. are performed in the reaction zones as a whole.
 15. Amethod according to any of claims 1 through 14, characterised in thatSiCl₄ obtained in step c) is fed back into the reaction of metallicsilicon with SiCl₄, H₂ and HCl (step a)).
 16. A method according to anyof claims 1 through 15, characterised in that SiH₄ generated in step c)is subjected to a distillative purification prior to its thermaldecomposition.
 17. A method according to any of claims 1 through 16,characterised in that silane and dichlorosilane-containing waste streamsof the distillation steps are collected to react with SiCl₄ to give atrichlorosilane-containing reaction mixture, with obtaining SiHCl₃ bydistillation from said reaction mixture.
 18. A method according to claim17, characterised in that in the reaction of the silane anddichlorosilane-containing waste streams with SiCl₄ liquiddisproportionation catalysts having a boiling point above the boilingpoint of SiCl₄ are used.
 19. A method according to any of claims 1through 18, characterised in that the thermal decomposition of SiH₄(step d)) on high-purity silicon seed particles is performed in a fluidbed at a pressure of 100 to 900 mbar.
 20. A method according to any ofclaims 1 through 19, characterised in that in the thermal decompositionof SiH₄ up to 50 vol. % of a silicon-free gas, relative to the overallquantity of the supplied gas, is added in addition to the SiH₄ from stepc).
 21. A method according to claim 20, characterised in that hydrogenis used as silicon-free gas.
 22. A method according to any of claims 1through 21, characterised in that H₂ generated in step d) is fed backinto the reaction of metallic silicon with SiCl₄, H₂ and HCl (step a)).23. A method according to any of claims 1 through 22, characterised inthat in the thermal decomposition of purified SiH₄ (step d)) high-puritysilicon powder is generated as by-product which is heated up to atemperature of at least 300° C. and agglomerated by means of microwaveirradiation in a wavelength range between 0.5 kHz and 300 GHz.
 24. Theutilisation of the high-purity silicon produced according to any of thepreceding claims for the production of solar cells.